Best way to share derivations and variables among modules?

[nxy7]

Hello, I was recently trying to share some state among my submodules and I thought that something like this could attach custom attributes to config.

    flake-parts.lib.mkFlake { inherit inputs; } {
      imports = [
        inputs.devshell.flakeModule
        (_: {
          perSystem = { config, self', inputs', pkgs, system, inputs, ... }:
            let
              overlays = [
                inputs.fenix.overlays.default
                (final: prev:
                  {
                    rustToolchain =inputs.fenix.packages.${system}.minimal.toolchain;
                  })
              ];
              rustToolchain = inputs.fenix.packages.${system}.minimal.toolchain;
            in {
              _module.args = {
                pkgs = import inputs.nixpkgs {
                  inherit system overlays;
                  config.allowUnfree = true;
                };
                rustToolchain = rustToolchain;
                buildRustPackage = (config._module.args.pkgs.makeRustPlatform {
                  cargo = rustToolchain;
                  rustc = rustToolchain;
                }).buildRustPackage;
              };
            };
        })
      ];

So basically what I thought I was doing was attaching rustToolchain and buildRustPackage to config so I could later access it via config.rustToolchain. This doesn't work, but it got me thinking - maybe it should work? Was I trying to achieve it in a bad way maybe? I imagine that sharing state like that between submodules could be useful and it would be pretty weird to use overlays to share config variables.
Are there any other ways to share derivations/variables between submodules or should I attach things to pkgs?

[nxy7]

Closing discussion as I've found examples of how it can be done in some configs on github